1. Field
The present disclosure relates to a manganese tin oxide-based transparent conducting oxide, a multilayer transparent conductive film using the same and a method for fabricating the same. More particularly, it relates to a manganese tin oxide-based transparent conducting oxide (TCO) with an optimized composition, which has low surface roughness, low sheet resistance and high transmittance even when deposited at room temperature, a multilayer transparent conductive film using the same and a method for fabricating the same.
[Description about National Research and Development Support]
This study was supported by the Ministry of Trade, Industry and Energy, Republic of Korea (Global Professional Technology Development Business, Project No. 1415135423) under the superintendence of Korea Evaluation Institute of Industrial Technology.
2. Description of the Related Art
Transparent conducting oxides (TCO) are important materials used in plasma panel displays (PDPs), light-emitting diodes, various touch panels, etc. Recently, interests in and researches on transparent electrodes are increasing with the development of optical devices, thin-film transistors, thin-film solar cells, etc. The research and commercialization of thin-film TCO materials began in the 1960s. In general, oxide-based materials in which SnO2, In2O3, ZnO, etc. are doped with dopants have been developed as optically transparent and conducting materials.
Among them, the Sn-doped In2O3 (ITO, indium tin oxide)-based thin film, which exhibits high work function, superior electrical conductivity, high transmittance, superior adhesion to a substrate and good etchability, is the most commercialized material mainly for flat panel displays (see Korean Patent Publication No. 2002-96536).
However, the worldwide reserve of indium (In), the main component of ITO, is low and indium (In) is expected to be depleted soon because the demand on ITO is increasing with the recent development of optical device industries. Therefore, the price competitiveness of the device has decreased. In addition, when the ITO thin film is deposited at low temperature, sheet resistance increases due to very high defect density because of decreased chemical stability. Accordingly, it is not suitable for fabrication through a low-temperature process and is inapplicable to flexible devices which are regarded as key display devices in the future. Because a plastic substrate with low thermal resistance is used for application to flexible devices, it is inapplicable to a high-temperature process because deformation occurs easily at a specific temperature or more.
For these reasons, researches have been conducted on materials that can replace ITO electrodes. Many researchers have studied transparent conducting oxide (TCO) materials not containing indium (In) such as Al-doped ZnO, Ga-doped ZnO, etc., which are abundant, pollution-free and thermally stable, to replace ITO.
However, up to now available non-indium-based transparent conducting oxides are inferior to ITO in its electrical and optical properties such as high resistance for room-temperature processes and requiring a film thickness of 200 nm or more to obtain an electrode with low resistance.
To resolve the problem of low electrical and optical properties of non-indium-based transparent conducting oxides for room-temperature processes, researches are actively under way on multilayer thin films with a structure of TCO/metal layer/TCO wherein a metal layer is disposed between transparent conducting oxide (TCO) thin films. The metal layer inserted in these multilayer thin films can decrease the electrical resistance of the overall thin film and may enhance transmittance through an anti-reflection effect of inhibiting reflection from the metal layer. As for the metal layer inserted in the multilayer thin film, silver (Ag) which absorbs less light in the visible region is used the most widely. When the metal layer inserted between the TCO layers is deposited in vacuo, the metal layer is deposited after forming islands. That is to say, it is known that, after the islands are formed first, then the islands are combined as a single uniform layer to form a continuous film. When the Ag thin film is deposited through magnetron sputtering, a continuous thin film is formed stably when the film thickness is 10 nm or more, while the film thickness increases, transmittance decreases. Therefore, the thickness of the metal layer of about 10 nm is applied.
Since the multilayer thin film is applied with such thin thickness, the interfacial roughness between the TCO and the metal layer greatly affects the electrical and optical properties of the multilayer thin film. When the transparent conducting oxide (TCO) thin film is in crystalline (e.g., single crystalline or polycrystalline) phase, the metal layer cannot be deposited uniformly due to large surface roughness (for example, RMS of several tens of nanometers), as shown in FIG. 1. As a result, electron mobility may be decreased and to this end electrical resistance may be eventually increased because the electron transport path in the metal layer is interrupted by the interface.